Re: [PATCHv5 4/8] zswap: add to mm/

[Cody P Schafer <cody@linux.vnet.ibm.com>]

On 02/18/2013 11:24 AM, Seth Jennings wrote:
> On 02/15/2013 10:04 PM, Ric Mason wrote:
>> On 02/14/2013 02:38 AM, Seth Jennings wrote:
> <snip>
>>> +/* invalidates all pages for the given swap type */
>>> +static void zswap_frontswap_invalidate_area(unsigned type)
>>> +{
>>> +    struct zswap_tree *tree = zswap_trees[type];
>>> +    struct rb_node *node, *next;
>>> +    struct zswap_entry *entry;
>>> +
>>> +    if (!tree)
>>> +        return;
>>> +
>>> +    /* walk the tree and free everything */
>>> +    spin_lock(&tree->lock);
>>> +    node = rb_first(&tree->rbroot);
>>> +    while (node) {
>>> +        entry = rb_entry(node, struct zswap_entry, rbnode);
>>> +        zs_free(tree->pool, entry->handle);
>>> +        next = rb_next(node);
>>> +        zswap_entry_cache_free(entry);
>>> +        node = next;
>>> +    }
>>> +    tree->rbroot = RB_ROOT;
>>
>> Why don't need rb_erase for every nodes?
>
> We are freeing the entire tree here.  try_to_unuse() in the swapoff
> syscall should have already emptied the tree, but this is here for
> completeness.
>
> rb_erase() will do things like rebalancing the tree; something that
> just wastes time since we are in the process of freeing the whole
> tree.  We are holding the tree lock here so we are sure that no one
> else is accessing the tree while it is in this transient broken state.

If we have a sub-tree like:
     ...
    /
   A
  / \
B   C

B == rb_next(tree)
A == rb_next(B)
C == rb_next(A)

The current code free's A (via zswap_entry_cache_free()) prior to 
examining C, and thus rb_next(C) results in a use after free of A.

You can solve this by doing a post-order traversal of the tree, either

a) in the destructive manner used in a number of filesystems, see 
fs/ubifs/orphan.c ubifs_add_orphan(), for example.

b) or by doing something similar to this commit: 
https://github.com/jmesmon/linux/commit/d9e43aaf9e8a447d6802531d95a1767532339fad 
, which I've been using for some yet-to-be-merged code.

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